(1) Field of the Invention:
The present invention relates to a photosensitive composition. More particularly, the present invention relates to a photosensitive composition having excellent moldability and compatibility and suitable for use particularly as a material for flexible printing plates and further flexographic printing plates.
(2) Description of the Prior Art:
A photosensitive layer of a photosensitive master printing plate, particularly a solid master printing plate, essentially comprises a polymer component comprising a base polymer or a reactive base polymer, a photoreactive monomer or photoreactive oligomer, a photopolymerization initiator and, if necessary, other additives. The photosensitive layer is produced, for example, by a dry film-forming method wherein a solvent is used as described in Japanese Patent Publication Nos. 46-26125, 50-32645 and 57-28485 and U.S. Pat. Nos. 4,481,280 and 4,464,456. When the production of a flexographic printing plate from a synthetic rubber elastomer is intended, a photosensitive composition free of any solvent or having a high concentration as described in U.S. Pat. No. 3,674,486 or No. 4,517,278 is subjected to a calender molding method or melt extrusion molding method.
The dry film forming method wherein a solvent is used has advantages that the molding is conducted at a relatively low temperature, that the composition is not exposed to a high temperature and that a printing plate material having a homogeneous composition can be obtained. On the other hand, this method has problems that since the solvent must be removed by drying, the composition is inclined to be modified in the drying step, the plate material is liable to be strained and a thick plate cannot be produced easily and that since a large amount of a photoreactive monomer or oligomer cannot be used for preventing the fluidization in the drying step, the composition is limited. In the solvent-free calender molding method or melt extrusion method, the composition of the printing plate material easily becomes heterogeneous or it is inclined to be modified by heat, since the molding is conducted under conditions comprising a high shear, high temperature and high viscosity. Further, a composition which can be molded at a relatively low temperature has a problem that the shape retentivity of the obtained printing plate material is insufficient at ambient temperature.
In addition, a photosensitive composition has usually an essential problem that the compatibility of the components constituting the composition with one another is limited by the complicated structure thereof and, therefore, the components must be selected in a narrow limited range.